In recent years, with improving the scale of integration of semiconductor, power consumption due to leakage currents in transistors when a circuit does not operate becomes an issue. Hence, a semiconductor integrated circuit having a low consumption power mode, which decreases the leakage currents by turning a power supply of the circuit off when the semiconductor integrated circuit is not used, has been developed.
However, when a supply of the power supply is stopped, in the semiconductor integrated circuit, an internal state thereof is cancelled exclusive of nonvolatile memories.
Therefore, it is necessary to retract held data to an external storage device or the like in advance of turning-off of the power supply. For example, a computer system described in Patent Literature 1 discloses a method of keeping a state of its components and a resumption function thereof after a power supply is disconnected completely. The computer system disclosed in Patent Literature 1 uses a nonvolatile storage device being external of a computer system core that stores status information for a suspend/resume function. The computer system disclosed in Patent Literature 1 reads an internal state by using a scanning latch in components of the computer system. The read internal state is saved in a retraction storage area and the power supply is disconnected.
In addition, Patent Literature 2 discloses a “computer system” which saves, by a request of a user, contents of a memory needed for return, information on an input/output device and whatnot into an external storage device and which returns them if necessary.
Patent Literature 3 discloses an “information processing unit” which is capable of improving stability of a system by solving mismatching of device management information before suspend processing and after resume processing. The information processing unit disclosed in Patent Literature 3 includes, as hardware, a CPU, a main memory, a nonvolatile memory, an interface section, expansion devices, a user interface section, and a bus. A suspend processing program, a resume processing program, a device management table and so on are stored in the nonvolatile memory. Before this system end, the suspend processing program saves at least the contents of the main memory to which a program has been loaded in the nonvolatile memory as suspend data. When the system starts, the resume processing program restores the suspend data saved in the nonvolatile memory to the main memory, detects one or more extension devices connected to the interface section, compares the detection result with the device management table, and deletes, from the device management table, device identification information of the extension device which exists only in the device management table.
Patent Literature 4 discloses a “semiconductor device” which is transferable at a high speed to a standby mode where power consumption is reduced while keeping internal information. The semiconductor device described in Patent Literature 4 comprises a latch circuit including a nonvolatile memory and a writing circuit, and is characterized by writing volatile data of the latch circuit in the nonvolatile memory in advance of turning-off of a power supply. The semiconductor device described in Patent Literature 4 can save data rapidly without requiring a complex transfer operation by adding nonvolatile memory cells.
Patent Literature 5 discloses a “portable terminal” which can provide comfortable operation environment by eliminating power disconnection immediately after starting. The portable terminal described in Patent Literature 5 comprises a main control part consisting of a CPU and an input/output device, a ROM storing a program for operating the main control part, a RAM storing a user application program, a main battery serving as a driving power source, a power-supply switch turning the power supply from the main battery on/off, and a voltage detection part detecting a voltage of the main battery to send voltage data to the main control part. The main control part executes the user application when the voltage of the main buttery becomes not less than a prescribed voltage within a predetermined time interval after turning-on power, and turns the power-supply switch off unless the voltage of the main battery becomes not less than the prescribed voltage within the predetermined time interval.